Thermal analysis based circuit design

ABSTRACT

Methods and apparatuses for circuit design to reduce power usage, such as reducing temperature dependent power usage, and/or to improve timing, such as reducing temperature dependent delay or transition time. At least one embodiment of the present invention reduces the power dissipation and improves the timing of an integrated circuit to optimize the design. A thermal analysis is used to determine the temperature dependent power dissipation of a circuit and the temperature distribution of the circuit resulting from dissipating the heat created by the temperature dependent power dissipation. Then, the components of the design are selectively transformed to reduce the power dissipation and to improve timing based on the temperature solution. The transformation may include placement changes and netlist changes, such as the change of transistor threshold voltages for cells or for blocks of the circuit chip.

RELATED APPLICATIONS

This present application is a divisional of U.S. patent application Ser.No. 12/053,453, filed on Mar. 21, 2008, now U.S. Pat. No. 7,941,779,issued on May 10, 2011, which is a divisional of U.S. patent applicationSer. No. 11/034,391, filed on Jan. 11, 2005, now U.S. Pat. No.7,366,997, issued on Apr. 29, 2008.

FIELD OF THE TECHNOLOGY

The invention relates to automated circuit design, and more particularlyto the automated transformation and optimization of a design of acircuit for improved timing and reduced power.

BACKGROUND

For the design of digital circuits (e.g., on the scale of Very LargeScale Integration (VLSI) technology), designers often employcomputer-aided techniques. Standard languages such as HardwareDescription Languages (HDLs) have been developed to describe digitalcircuits to aid in the design and simulation of complex digitalcircuits. Several hardware description languages, such as VHDL andVerilog, have evolved as industry standards. VHDL and Verilog aregeneral-purpose hardware description languages that allow definition ofa hardware model at the gate level, the register transfer level (RTL) orthe behavioral level using abstract data types. As device technologycontinues to advance, various product design tools have been developedto adapt HDLs for use with newer devices and design styles.

In designing an integrated circuit with an HDL code, the code is firstwritten and then compiled by an HDL compiler. The HDL source codedescribes at some level the circuit elements, and the compiler producesan RTL netlist from this compilation. The RTL netlist is typically atechnology independent netlist in that it is independent of thetechnology/architecture of a specific vendor's integrated circuit, suchas field programmable gate arrays (FPGA) or an application-specificintegrated circuit (ASIC). The RTL netlist corresponds to a schematicrepresentation of circuit elements (as opposed to a behavioralrepresentation). A mapping operation is then performed to convert fromthe technology independent RTL netlist to a technology specific netlist,which can be used to create circuits in the vendor'stechnology/architecture. It is well known that FPGA vendors utilizedifferent technology/architecture to implement logic circuits withintheir integrated circuits. Thus, the technology independent RTL netlistis mapped to create a netlist, which is specific to a particularvendor's technology/architecture.

One operation, which is often desirable in this process, is to plan thelayout of a particular integrated circuit and to control timing problemsand to manage interconnections between regions of an integrated circuit.This is sometimes referred to as “floor planning.” A typical floorplanning operation divides the circuit area of an integrated circuitinto regions, sometimes called “blocks”, and then assigns logic toreside in a block. These regions may be rectangular or non-rectangular.This operation has two effects: the estimation error for the location ofthe logic is reduced from the size of the integrated circuit to the sizeof the block (which tends to reduce errors in timing estimates), and theplacement and routing typically runs faster because as it has beenreduced from one very large problem into a series of simpler problems.

A typical integrated circuit is designed for implementation on a chipsubstrate. For example, a silicon substrate can have devices, such asstandard cells, custom blocks, etc., formed on the substrate andinterconnected by layers of metal which are separated by layers ofdielectric materials.

A silicon chip dissipates power during operation. The temperature of thesilicon chip rises while the heat generated from the power used by thecircuit is being conducted away to the surrounding environment. Thesilicon substrate conducts away a portion of the heat, while the metalinterconnect stack above the silicon substrate used for signalinterconnect and power distribution conducts away a portion of the heat.

When the chip reaches a steady state for heat dissipation, at anelevated silicon temperature field equilibrium exists between the heatgenerated and the heat transported away by the temperature gradientcreated by the difference between the silicon temperature and theambient temperature. The silicon temperature field is typicallynon-uniform. The temperature in the silicon chip typically variesaccording to the locations, depending on the local heat generation andthe physical layout of the chip.

High temperature in an Integrated Circuit (IC) chip may reduce thereliability of the IC chip. Large temperature gradient in the chip maycause malfunction. For example, K. Banerjee, M. Pedram and A. H. Ajami(2001), “Analysis and optimization of thermal issues in high-performanceVLSI,” Proc. of 2001 International Symposium on Physical Design (ISPD),provide an overview of various thermal issues in high-performance VLSIwith especial attention to their implications for performance andreliability.

Some techniques have been developed to consider the steady state of thethermal conduction during the process of partitioning and placement.Examples of such techniques include: C. H. Tsai and S-M Kang (1999),“Standard Cell Placement for even on-chip thermal distribution,” Proc.of 1999 International Symposium on Physical Design (ISPD); Guoqiang Chenand Sachin Sapatnekar (2003), “Partition-Driven Standard Cell ThermalPlacement,” Proc. of 2003 International Symposium on Physical Design(ISPD); Kyoung Keun Lee, Edward J. Paradise, Sung Kyu Lim (2003),“Thermal-driven Circuit partitioning and Floorplanning with PowerOptimization,” Georgia Institute of Technology, Center for ExperimentalResearch in Computer Systems, Technical Reports, GIT-CERCS-03-07. Thesetechniques seek for an even temperature distribution in a partitioningand placement process during the early stage of a circuit design (e.g.,floorplanning to breaking down a circuit design into blocks and lay outthe blocks).

Some techniques for design optimization include three-dimensionalintegration. For example, S. Das, A. Chandrakasan and Rafael Reif(2004), “Timing, energy and thermal performance of three-dimensionalintegrated circuits,” Proc. of Great Lakes Symposium on VLSI (GL VLSI),show a method of combining multiple device layers with a high-densityinter-layer interconnect for 3-D integration of a given circuit toprovide better timing and energy performance relative to a single waferimplementation of the same circuit.

SUMMARY OF THE DESCRIPTION

Methods and apparatuses for circuit design to reduce power usage, suchas reducing temperature dependent power usage, and/or to improve timing,such as reducing temperature dependent delay or transition time, aredescribed here. Some embodiments of the present invention are summarizedin this section.

At least one embodiment of the present invention reduces the powerdissipation and improves the timing of an integrated circuit to optimizethe design. A thermal analysis is used to determine the temperaturedependent power dissipation of a circuit and the temperaturedistribution of the circuit resulting from dissipating the heat createdby the temperature dependent power dissipation. Based on the temperaturesolution, the components of the design are selectively transformed toreduce the power dissipation and to improve timing. The transformationmay include placement changes and netlist changes, such as the change oftransistor threshold voltages for standard cells or for blocks of thecircuit chip.

In one aspect of the present invention, a method implemented on a dataprocessing system for circuit design, includes: determining for a firstdesign of a circuit a first temperature solution and a first powerdissipation solution, where the first power dissipation solution and thefirst temperature solution are interdependent; and, transforming thefirst design of the circuit into a second design of the circuit usingthe first temperature solution to reduce power dissipation of thecircuit under one or more design constraints.

In one example of an embodiment, the first temperature solution iscomputed to represent a steady state thermal dynamic balance between thefirst design of the circuit and a surround environment of the circuit.The first power dissipation solution includes temperature dependentpower dissipation of elements of the first design. The temperaturedependent power dissipation includes leakage power of elements of thefirst design.

For example, the transformation may include: moving an element of thefirst design of the circuit from a first location which has a firsttemperature according to the first temperature solution to a secondlocation which has a second temperature according to the firsttemperature solution, in which the first temperature is higher than thesecond temperature.

In one example of an embodiment, the first design includes a placementsolution and a routing solution. In one example, the transformationreduces leakage power of the circuit under the one or more designconstraints, such as a timing constraint.

In one example of an embodiment, the transformation includes replacing afirst element of the first design of the circuit with a second elementof the second design of the circuit. The first and second elements arefunctionally equivalent. The second element has power dissipation lowerthan the first element at a same temperature. In one example, the firstelement has a first transistor threshold voltage; the second element hasa second transistor threshold voltage; and the first transistorthreshold voltage is lower than the second transistor threshold voltage.In one example, the second element is smaller than the first element.

In one example of an embodiment, the transformation includes: replacinga third element of the first design of the circuit with a fourth elementof the second design of the circuit to meet a timing requirement on apath passing through the first and third elements. The third and fourthelements are functionally equivalent. The fourth element has powerdissipation higher than the third element at a same temperature. In oneexample, the third element has a third transistor threshold voltage; thefourth element has a fourth transistor threshold voltage; and the fourthtransistor threshold voltage is lower than the third transistorthreshold voltage.

In one example of an embodiment, the first and third elements areselected on a signal path of the first design of the circuit; the firstelement is at a first location which has a first temperature accordingto the first temperature solution; the third element is at a secondlocation which has a second temperature according to the firsttemperature solution; and the first temperature is higher than thesecond temperature.

In one example of an embodiment, the method further includes:determining a timing critical path; and selecting a first element of thetiming critical path based on at least the first temperature solution;and the transformation includes replacing the first element with asecond element to improve timing on the timing critical path. In oneexample, the first element is selected from a low temperature portion ofthe timing critical path; and the second element has a transistorthreshold voltage lower than the first element.

In one aspect of the present invention, a method implemented on a dataprocessing system for circuit design, includes: determining atemperature solution and a power dissipation solution for a first designof a circuit; and transforming the first design of the circuit into asecond design of the circuit using the temperature solution to improveperformance of first design of the circuit. The power dissipationsolution and the temperature solution are interdependent; and the powerdissipation solution and the temperature solution are consistent witheach other

In one example of an embodiment, the method further includes: selectingan element of the first design from a location with an elevatedtemperature according to the temperature solution. The first design istransformed into a second design through replacing a firstimplementation of the element with a second implementation to reduceleakage power. In one example, the first implementation has a transistorthreshold voltage lower than the second implementation. In one example,the method further includes: selecting an element of the first designfrom a location with a low temperature according to the temperaturesolution. The first design is transformed into a second design throughreplacing a first implementation of the element with a secondimplementation to improve timing on a path while limiting an increase inleakage power. In one example, the second implementation has atransistor threshold voltage lower than the first implementation.

In one aspect of the present invention, a method implemented on a dataprocessing system for circuit design, includes: determining atemperature solution and a power dissipation solution for a first designof a circuit, where the power dissipation solution and the temperaturesolution are interdependent and consistent with each other; andperforming timing optimization using the temperature solution togenerate a second design of the circuit. In one example, the methodfurther includes: evaluating a temperature dependent delay from an inputof an element to an output of the element. In another example, themethod further includes: evaluating a temperature dependent transitiontime of an element at an output of the element. The element, forexample, can be a standard cell; a custom block; or a custom macro.

In one example of an embodiment, the first design of the circuitincludes a technology dependent netlist in a Hardware DescriptionLanguage (HDL). In one example, the timing optimization is performedtogether with power optimization. In one example, the timingoptimization includes netlist changes such as changing a transistorthreshold value for a cell instance or a block.

In one aspect of the present invention, a method implemented on a dataprocessing system for circuit design, includes: selectively changingtransistor threshold value for at least a portion of a design of acircuit to optimize power usage under timing constraint. In one example,the transistor threshold value is selectively changed for individualcell instances. In another example, the transistor threshold value isselectively changed for blocks of a chip. In one example, selection ofthe portion of the design is at least partially dependent on atemperature solution for the design of the circuit; the temperaturesolution is based on heat transfer according to a temperature dependentpower dissipation solution for the design of the circuit. In oneexample, the method further includes: determining the temperaturesolution and the temperature dependent power dissipation solution, whichare interdependent and consistent with each other.

The present invention includes methods and apparatuses which performthese methods, including data processing systems which perform thesemethods, computer readable media which when executed on data processingsystems cause the systems to perform these methods, circuits designedusing these methods, and computer readable media which contains the datarepresentation of the circuits designed using these methods.

Other features of the present invention will be apparent from theaccompanying drawings and from the detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings in which likereferences indicate similar elements.

FIG. 1 illustrates a circuit design process for the optimization ofpower dissipation and timing according to one embodiment of the presentinvention.

FIG. 2 illustrates a method to optimize a circuit design according toone embodiment of the present invention.

FIG. 3 illustrates a method to obtain a self-consistent thermal fieldfor the optimization process according to one embodiment of the presentinvention.

FIG. 4 illustrates a detailed example of optimizing a circuit designaccording to one embodiment of the present invention.

FIG. 5 illustrates one example transformation to optimize a circuitdesign according to one embodiment of the present invention.

FIG. 6 illustrates another example transformation to optimize a circuitdesign according to one embodiment of the present invention.

FIG. 7 illustrates a further synthesis transformation to optimize acircuit design according to one embodiment of the present invention.

FIG. 8 illustrates a detailed flow of synthesis transformations tooptimize a circuit design according to one embodiment of the presentinvention.

FIG. 9 shows a block diagram example of a data processing system whichmay be used with the present invention.

DETAILED DESCRIPTION

The following description and drawings are illustrative of the inventionand are not to be construed as limiting the invention. Numerous specificdetails are described to provide a thorough understanding of the presentinvention. However, in certain instances, well known or conventionaldetails are not described in order to avoid obscuring the description ofthe present invention. References to one or an embodiment in the presentdisclosure are not necessarily references to the same embodiment; and,such references mean at least one.

At least one embodiment of the present invention reduces the powerdissipation and improves the timing of an integrated circuit to optimizethe design. After computing the power dissipation in an original design,the temperature across the chip due to the heat generated is computed.Then, the components of the design are selectively transformed to reducethe power dissipation and to improve timing based on the temperaturesolution. For example, cells with high power dissipation and/or largertemperature dependent portion of delays can be moved away from hightemperature regions into low temperature regions to reduce the overallpower dissipation and/or improve timing.

In one embodiment of the present invention, the power dissipation,temperature and the design (e.g., placement and netlist) areinterdependent. Transforming the design causes the changes in the powerdissipation distribution which changes the temperature distribution,where the power dissipation distribution depends on the temperaturedistribution.

In one embodiment of the present invention, a thermal analysis is usedto minimize the sources of power that are most directly affected bytemperature: leakage power and cell switching power. Significantreduction in power can be achieved without over-constraining the designwhich may lead to difficultly in meeting the timing requirements.

In one embodiment of the present invention, explicit models for thetemperature variation of the power and/or delay are used for the thermalanalysis based optimization.

In one embodiment of the present invention, both power and timing areoptimized simultaneously based on the thermal analysis.

FIG. 1 illustrates a circuit design process for the optimization ofpower dissipation and timing according to one embodiment of the presentinvention. In FIG. 1, a circuit design (201) is transformed to improvethe performance of the circuit using a thermal model (203) of thecircuit design (201). For example, the circuit design may be transformedfor reduced power dissipation and/or improved timing. Improving timingperformance on critical paths can lead to a reduced clock period so thatthe circuit can run at a high frequency.

In one embodiment of the present invention, the thermal model (203)represents the steady state of the heat transfer between the chipimplementing the circuit design (201) and the surrounding environment(e.g., ambient temperature). The circuit design (201) includes blocks(e.g., 211, 213, 215, . . . ), which consume power when in operation.The consumed power of the circuit blocks, such as standard cells, customdesigned blocks, or blocks of cells, are represented as the heat sources(e.g., 231, 233, 235, . . . ) in the thermal model.

According to one embodiment of the present invention, a thermal analysis(221) is performed based on the circuit design (201) to determine theself consistent power dissipation and temperature solution. At least onedesign transformation (223) is then performed based on the thermalanalysis. For example, the design transformation (223) can include asynthesis transformation to replace one or more blocks with one or morefunctionally equivalent corresponding blocks to optimize a design goal.For example, the block may be transformed to reduce the leakage power,to improve timing performance, etc.

FIG. 2 illustrates a method to optimize a circuit design according toone embodiment of the present invention. Operation 301 determines atemperature distribution of an Integrated Circuit (IC) chip and a powerdissipation distribution of the IC, where the power dissipationdistribution and the temperature distribution are interdependent andconsistent with each other. If operation 303 determines that it isdesirable to perform a design transformation to reduce the powerdissipation, operation 305 performs one or more design transformationsto reduce power dissipation (e.g., reduce leakage power) under designconstraints (e.g., timing constraints). Operation 307 determines whetherthe power dissipation distribution is changed significantly due to thedesign transformations. If the power dissipation distribution has notbeen changed significantly, the temperature solution can be further usedto perform the design transformation; otherwise, operation 301 isperformed to update the temperature solution and the power dissipationdistribution. Operations 301-307 can be repeated until a desired designgoal is achieved (e.g., the power usage is under a given limit or thereis no significant improvement).

Typically, the power dissipation of an Integrated Circuit (IC) chipchanges as the computation task performed by the IC chip changes. Anumber of techniques have been published for the estimation of powerdissipation in a chip to resolve the uncertainty (or time dependency) inthe computation tasks. See, for example, F. Najm (1994), “A survey ofpower estimation techniques in VLSI circuits,” IEEE Transactions on VLSISystems, Vol. 2, No. 4, pp. 446-455. Some of the techniques useprobabilistic methods; and some of the techniques use a moredeterministic approach by applying input vectors and running logicsimulations to obtain a large set of switching activities and toestimate the average switching at cell output. These techniquestypically generate a form of average computation tasks for thedetermination of the computation dependent part of the power dissipationfor the cell instances.

For the design purpose, the power dissipation estimated for the circuitblocks is typically based on a form of average computation activities.The exact power usages in the circuit blocks are typically based on thecomputation tasks of the circuit, which typically varies over the timein real world usage. An estimation of the computation activities canthen be used to determine the power dissipation for various blocks ofthe circuit design. For example, the computation activities can beestimated in terms of frequency of switching activities, probabilitiesof state switching, or combinations of different input vectors, etc.which typically lead to an estimation of a form of average powerdissipation due to computation activities over time.

In general, when the time dependent aspect of the power dissipation andtemperature variation is not of particular interest during the designprocess, a form of power dissipation averaged over time can be used toobtain a “typical” temperature distribution. Further, a steady state ofthe heat dissipation can be used to obtain a “typical” temperaturedistribution and remove the time varying aspect of the heat dissipationprocess.

Thus, in one embodiment, a form of the computation activity usage of thecircuit blocks is used to remove the time varying aspect of thecomputation activities; and a steady state solution for the thermalmodel is use to remove the time varying aspect of the time varyingaspect of the heat transfer process. The power dissipation solution andthe temperature field solution may or may not be exactly equal to a realworld power dissipation distribution and a real world temperature fieldfor an implementation of the circuit design.

Thus, the temperature field solution to the thermal model (205)represents a time independent aspect of the temperature field of thecircuit design implemented as an Integrated Circuit (IC) chip. An ICchip is typically made of a silicon substrate with metal layers forinterconnect and other materials, such as dielectric materials thatseparates the metal layers and package materials which protect the ICchip.

In one embodiment of the present invention, the power dissipations ofthe circuit blocks (e.g., 211, 213, 215) are based on not only thecomputation activities but also the temperature of the circuit blocks.Thus, the temperature solution and the power dissipation solution areinterdependent. In one embodiment, the power dissipations of the circuitblocks include a temperature dependent portion, which is optimized usingthe temperature solution obtained from the thermal model (203) based onthe temperature dependent power dissipation solution.

In one embodiment of the present invention, the temperature dependentpower dissipation solution and the temperature solution based on thepower dissipation solution are consistent with each other. In oneaspect, the temperature field used to determine the power dissipationsolution is consistent with the temperature solution obtained fromsolving the heat dissipation problem according to the determined powerdissipation solution. In another aspect, the power dissipation used todetermine a temperature solution is consistent with the powerdissipation solution computed according to the temperature solution.

FIG. 3 illustrates a method to obtain a self-consistent thermal fieldfor the optimization process according to one embodiment of the presentinvention. After operation 321 determines an initial temperaturedistribution of an Integrated Circuit (IC) chip, operation 323determines the power dissipation of cell instances of the IC based onthe temperature distribution. The initial temperature distribution maybe a uniform temperature field at the ambient temperature or otherpredicted temperatures.

Operation 325 interpolates the power dissipation in the IC chip toobtain a power dissipation distribution. Typically, the thermal model ofthe continuum media for heat transfer (e.g., in the form of a Laplace'sequation) is discretized using a numerical scheme, such as a finitedifference method, a finite element method, a boundary element method,etc. Some of the numerical methods may not require the explicitinterpolation of the power dissipations, since the power dissipation ofthe cell instances can be directly used.

Operation 327 calculates a new temperature distribution based on thepower dissipation distribution (e.g., through solving the heat transferequation using a numerical scheme). Operation 329 determines whetherconsistency is achieved. If the new temperate distribution is consistentwith the previous temperate used to determine the power dissipationdistribution of the thermal model (or the power dissipation distributioncomputed using the new temperature distribution is consistent with theprevious iteration of the power dissipation distribution), consistencyis achieved. Otherwise, operations 323-329 can be repeated untilconsistency is achieved.

In one embodiment of the present invention, the power dissipation inintegrated circuits includes three major components: net switching power(P_(net)), cell switching power (P_(switching)) and cell leakage power(P_(leakage)), where the cell may be a standard cell or a larger customdesigned block whose behavior is understood. The switching power is thepower dissipated for signal/state switching in the net or in the cell.The cell leakage power is due to the leakage current, which can arisefrom reverse bias diode currents and sub-threshold effects. A detaileddiscussion on leakage current can be found in K. Roy, S. Mukhopadhyay,H. Mahmoodi-Meimand (2003), “Leakage current mechanisms and leakagereduction techniques in deep-submicrometer CMOS circuits,” Proceedingsof the IEEE, Vol. 91, No. 2, pp. 305-327.

The net switching power typically does not depend on the silicontemperature of the silicon, because it is primarily controlled by theload capacitance (wire or gate capacitance) and the supply voltage. Thecell switching power and the leakage power typically increase withsilicon temperature. When the temperature increases, the leakage powertypically increases more rapidly than the cell switching power. The cellswitching power typically increases linearly (or according to apolynomial function) with respect to the temperature. The leakage powertypically is an exponential function of the silicon temperature.

In one embodiment of the present invention, the temperature variation ofpower dissipation is modeled based on the physical phenomenon itrepresents. In one implementation, P_(net) is independent of thetemperature; P_(switching) is a slowly increasing function of thetemperature, such as P₀+a×T+b×T², where P₀ is independent on thetemperature; and P_(leakage) is modeled as an exponential function oftemperature, such as c×e^(kT). However, it is understood that theanalysis can be performed with any computational model for thetemperature dependency in general.

In the silicon chip, the power dissipation causes the elevatedtemperature. The elevated temperature then in turn creates more powerdissipation due to increased leakage and switching power. The elevatedtemperature also causes larger temperature gradient, which speeds up theheat dissipation. Thus, at a given point on the chip there exists alocal equilibrium temperature at which the circuit operates with anelevated steady state temperature.

When the integrated circuit devices continue to reduce the feature size,such as scaling into deep submicron technologies, the leakage powerbecomes a significant part of the total power dissipation for advancedprocess technologies. For example, when the process technologies advanceto the feature size in the order of 90 nm, 65 nm, or smaller, theleakage power becomes a dominant part of the total power dissipation.Minimizing the power dissipation due to leakage current cansignificantly reduce the total power usage.

One embodiment of the present invention uses the result of the thermalanalysis to optimize the circuit design under various design constrains.For example, synthesis transformations are automatically performed toimprove a design goal, which may be a combination of area usage, clockfrequency, power usage, etc. For example, power usage or leakage powercan be minimized for a specified clock frequency and power supplyvoltage. Similarly, the clock frequency or other timing parameters canbe improved with least increase in power usage.

In one aspect, the optimization of a circuit design includes theminimization of power usage, such as the cell switching and leakagecomponents of the total power dissipation, without reducing the clockfrequency or the power supply voltage and with minimal additional areapenalty.

In another aspect, the optimization of a circuit design includes thetiming optimization (e.g., to reduce the clock frequency) using thetemperature distribution information without significant penalty inpower usage.

In one embodiment of the present invention, a synthesized and placednetlist is optimized for a target frequency and a specified supplyvoltage. For example, the netlist can be a technology dependent netlist.The information regarding the temperature dependent of power dissipationfor the elements of the circuit, such as the standard cells and customblocks, is used for the thermal model. The optimization includesimproving the timing to meet the target frequency and/or reducing thepower dissipation of the circuit.

In one embodiment of the present invention, a power minimization processincludes: determining self-consistently the local power dissipation andthe local temperature across the chip; performing optimizations on thedesign by placement changes or netlist changes; re-computing thetemperature and power to assure that the optimizations is based on theself-consistent local power dissipation and local temperature.

For example, components whose power dissipation is highly sensitive tothe local temperature can be moved from elevated temperature areas tolow temperature areas. Further, low power dissipation implementationscan be used to replace high power dissipation implementations in areasof elevated temperature.

In general, a logic function can be implemented in different ways andusing different cells, which provide the trade-off between the powerdissipation and other design performances. The use of low powerdissipation implementations typically has negative impact on otherdesign parameters. For example, standard cells with high transistorthreshold values have low leakage power but reduced timing performance.A low power dissipation implementation of a logic function can take alarger area and/or longer delay than a high power dissipationimplementation of the same logic function.

In one embodiment of the present invention, the blocks in the hightemperature regions are transformed to trade certain performance forreduced power dissipation, especially for highly temperature dependentcomponents. The blocks in the low temperature regions can then betransformed to gain back the corresponding performance to satisfy thedesign constraints, such as timing, area usage, etc.

If the design transformation perturbs significantly the thermalcondition, such as the power dissipation or the temperaturedistribution, the self-consistent local power dissipation and the localtemperature across the chip are recomputed. The iterations ofdetermining self-consistent thermal solutions and performingoptimizations can be repeated until a desired design goal is achieved(or when no significant improvement is achieved).

In one embodiment of the present invention, the temperature distributionbased on the power dissipation of the elements of the circuit in designis determined from the thermal model of the IC chip. In oneimplementation, the thermal model of an IC chip is in the form of thefollowing Laplace's equation:∇(K(x,y,z)∇T)=−P(x,y,z,T)

where P(x, y, z, T) includes the contribution from the net switchingpower (P_(net)), the cell switching power (P_(switching)) and theleakage power (P_(leakage)). The power dissipation at the cell instancescan be smoothly interpolated over the IC chip. K is the thermalconductivity at a given location and is based on the material present atthe location. For example, in the silicon region the thermalconductivity of silicon is used; and in the dielectric above the siliconthe corresponding thermal conductivity for the dielectric material isused.

In one implementation, the power dissipation sources are assumed to bein the plane of the silicon because the transistors that dissipate mostof the power are at there. However, when more detailed designinformation is available (e.g., the metal layer routing information),the locations of the power dissipation sources can be modeled moreprecisely. For example, the net switching powers may be modeled in theplanes of the metal layers.

The thermal model includes appropriate boundary conditions, whichdepends on the packaging used and the assumed working environment, suchas the ambient temperature. The thermal model is solved to yield atemperature distribution for the whole chip, which can be used to updatethe power source of the thermal model.

In one embodiment of the present invention, an initial temperaturedistribution T₀ is assumed for the computation of the power sourceP(T₀). The power source P(T₀) is then used to determine the temperaturedistribution T₁, which is then further used to obtained the updatedpower source P(T₁). The iteration continues until the solution convergesto a self consistent temperature and power source solution.

In one embodiment of the present invention, when a power source P isdetermined based on an initial temperature distribution or thetemperature solution of the previous iteration, the thermal model issolved using one of many available numerical techniques, such as finitedifferencing, finite elements, boundary value methods, etc.

The temperature distribution T_(i+1) obtained by solving the Laplace'sequation will not in general be the same as the original temperaturedistribution T_(i) used to calculate the power distribution. Theiteration on the power dissipation calculation and the temperaturecalculation can be performed until the temperature distribution does notchange any further (or the power source does not change any further).

In one embodiment of the present invention, once a self consistenttemperature and power dissipation solution is obtained from a thermalanalysis, design transformations are preformed to improved the design ofthe circuit using the thermal solution. For example, transformations canbe performed to reduce the leakage power of the circuit.

In one embodiment, the cells that have high leakage current at a hightemperature region are selected for transformation to minimize power.The leakage current can have an exponential dependence on thetemperature. When the cells with high leakage current are moved to a lowtemperature region, the leakage current can be reduced significantly.

For example, in one embodiment, placement modification techniques areused to move cells with high leakage or switching power from hightemperature areas to lower temperature areas to reduce power usage. Forexample, in an area with a large temperature gradient, a cell can berelocated from a high temperature region to a low temperature regionover a short distance. Such a placement change can reduce thetemperature gradient and the power usage without significant impact ontiming, area usage, etc.

However, changing placement over a long distance can cause significantchanges to the circuit design, which can be computationally intensive inan optimization process. In one embodiment of the present invention,netlist changes are also used to reduce the leakage power withoutsignificant changes in placement over a long distance.

In one embodiment, some transformations are performed without moving anelement from one temperature zone to another temperature zone.Transformations for netlists in different temperatures can be performedto reduce power usage without violating other design constraints.

For example, the library of standard cells can contain cells with thesame functionality but different leakage power, such as multiplethreshold voltage transistors. Different standard cells with differenttransistor threshold voltages may be provided for the implementation ofthe same functionality of an element of a circuit. Thus, the designtransformation can involve selecting the standard cells for reducedpower usage while maintaining other design constraints. For example, ina high temperature region low leakage power cells that have a hightransistor threshold voltage can be used to replace the high leakagepower cells that have a low transistor threshold voltage. If the timingconstraint for the chip allows the use of the high-threshold voltagecells, the high-threshold voltage cells can be used in the hightemperature area to reduce leakage power.

In general, high-threshold cells are slower so that they are usedsparingly to meet the timing constraints. In one embodiment of thepresent invention, the high-threshold cells are used most effectivelythrough the use of the criterion of applying the high-threshold cells inthe high temperature regions.

Low leakage power implementations are typically slower and/or largerthan high leakage power implementations. For example, the use of the lowleakage power cells with a high transistor threshold voltage in the hightemperature region can degrade the timing performance on a criticalpath.

In one embodiment of the present invention, to compensate thedegradation in the timing performance, one or more elements on the lowtemperature portion of the critical path is also transformed to tradepower dissipation for timing performance. For example, the high leakagepower, high performance cells can be used in a low temperature region ofthe critical path to replace the low leakage power, low performancecells on the critical path so that the timing constraint is notviolated. When the standard cell library includes multiple versions ofcells for the same functionality, the temperature solution can be usedto determine the selection of cells along timing critical path for theoptimization of power usage and timing.

Since the leakage power depends on the temperature exponentially, theuse of high leakage power cells in a low temperature region in exchangefor the use of low leakage power cells in a high temperature region tomaintain the timing performance can lead to reduction in overall leakagepower.

In one embodiment, a synthesis transformation is performed to convertone implementation of a logic block to another without relocating thelogic block to a different location. Typically, a logic block can besynthesized into different logic implementations with differentcharacteristics (e.g., leakage power usage, area usage, timingperformance, etc.) In one embodiment of the present invention, to reducethe power usage one logic block at a high temperature region isselectively transformed for a low power implementation. To avoidviolating other design constraints (e.g., area usage or timing), the lowtemperature region can be transformed accordingly to trade power (and/orarea usage) for performance. Thus, the overall performance of the designis improved.

The use of a low power version reduces the power usage even if thetemperature of the location remains the same. Actually, the reduction inthe power dissipation can lead to the reduction in local temperature,which can further reduce the power usage.

If the fraction of cells transformed is small, the overall temperaturedistribution may not change significantly by the changes and can stillbe used for the optimization.

FIG. 4 illustrates a detailed example of optimizing a circuit designaccording to one embodiment of the present invention. Operation 341divides a chip into a plurality of regions (or bins). Operation 343computes average temperatures for the regions. Operation 345 sorts theregions according to the average temperatures. Operation 347 processesthe region with the highest average temperature.

Operation 349 sorts the instances in the region according to the leakagepowers. Alternatively, the instances can be sorted according to thesensitivity of leakage power to temperature (e.g., d P_(leakage)/d T) ora combination of leakage power and sensitivity of leakage power totemperature. Operation 351 performs one or more design transformationsfor the instance with the highest leakage powers to reduce the leakagepower without violating design constraints (e.g., timing and/or wiringrequirements). For example, if a placement tool uses a force directedalgorithm to place the instances to minimize congestion, then thetemperature can be used in an analogous manner to the local utilizationin the cost function of the force directed placer to produce a more eventemperature distribution and lower the total power as well. Thesensitivity would be used in the selection criteria of the forcedirected placer to identify which instances to move preferentially. Ifoperation 353 determines to process the next instance, operation 351 isperformed to further transform the next highest leakage power cell.

In one embodiment, the process of operations 351 and 353 repeats untilthe moved instances in the hottest region causes the temperature in theblock is reduced or no further changes can be made due to timing orwiring requirements. If operation 355 determines to process the nextregion, operations 349-353 are repeated for the next highest temperatureregion. In one embodiment, the process of operations 349-355 repeats forregions until the power usage meets the specification or the top half ofthe sorted bins have been analyzed or no significant improvement isachieved.

In one embodiment, the power dissipation in the transformed regions isevaluated to determine if the temperature solution needs to be updated.If the temperature solution is updated, operations 341-355 can berepeated.

In one embodiment, the cells selected from the high temperature regionsare moved to the lowest temperature region among target regions where ifthe cells are moved to the timing and wiring requirements are stillsatisfied.

FIG. 5 illustrates one example transformation to optimize a circuitdesign according to one embodiment of the present invention. In FIG. 5,the chip according to a design 403 is divided into regions, such asregion 421, region 423, etc. FIG. 5 illustrates a way to divide the chipinto regions using a regular mesh. In general, the regions can beirregular in shapes and arrangement. For example, the regions can bedivided according to the contour lines of the temperature distributions.The regions can be divided so that the temperature within the regions issubstantially uniform.

In one embodiment, the regions are sorted according to their averagetemperatures. The regions are processed in the order of their averagetemperature, starting from high temperature regions to low temperatureregions. The cell instances in a region are sorted according to thetemperature dependent portion of the power or the leakage power. Forexample, in region 421, cell instances 411, 413, 415, etc., areprocessed according to the leakage power of the cells. The cellinstances with high leakage power are processed first.

For example, if the cell instance 411 has the highest leakage power inthe region 421, the cell instance 411 may be moved to a lowertemperature region. The regions that have temperatures lower than theregion 421 are the candidate regions of the move.

Typically, a subset of the candidate regions are allowable, since movingthe cell instance 411 to a remote region may cause too much delay intiming to satisfy the timing constraint or cause area constraintviolation in a congested area or cause violation in routing rules. Theavailability of routing resources can also limit the target regions thecell instance can be moved into. Thus, the set of allowable targetregions can be determined so that the cell instance can be moved intoone of the allowable target regions without violating a designconstraint.

From the set of allowable target regions, a region with the lowesttemperate can be select as the target region. For example, when theregion 423 is selected as the target region, the design transformation401 moves the cell instance 411 from the source region 421 into thetarget region 423 to generate the new design 405.

FIG. 6 illustrates another example transformation to optimize a circuitdesign according to one embodiment of the present invention. In FIG. 6,a path 503 includes a high leakage cell instance 519 in a high temperateregion and a low leakage cell instance 515 in a low temperature region.The path is between registers 511 and 523, passing through logic 513,cell instance 515, logic 517, cell instance 519, and logic 521.

When the path 503 is not timing critical, the high leakage cell instance519 having a leakage power P_(A) at temperature T₁ can be replaced witha low leakage cell instance such as cell instance 539 which perform thesame function. For example, the cell instance 539 has a highertransistor threshold voltage than the cell instance 519 so that the cellinstance 539 has a reduced leakage power (e.g., P_(C)<P_(A) attemperature T₁) even if the temperature of the cell instance remains thesame (e.g., T₁). However, the use of the high threshold instancedegrades the timing performance and reduces the slack of the path. Ifthe timing requirement is still satisfied, such an design transformationis acceptable.

When the path is timing critical (or sub-critical), the use of the highthreshold instance can cause timing violation. In one embodiment of thepresent invention, the cell instance 515 having a leakage power P_(B) inthe low temperature region of temperature T₂ is also transformed toimprove the timing performance and avoid timing violation. For example,a low threshold instance 535 with a leakage power P_(D)>P_(C) attemperature T₂ is used to replace the high threshold instance 515. Thus,when both the cell instances in the high temperature region and in thelow temperature region are transformed to convert path 503 into path505, the timing constraint remains satisfied.

Note that cell instances 519 and 535 may not perform the same functionand may not have the same resistor threshold voltage.

Although the use of low threshold instance in the low temperature regionincreases the leakage power, the overall transformation (501) for bothcell instances 515 and 519 reduces the leakage power. Since the leakagepower is highly sensitive to the temperature, the leakage powerreduction in the high temperature region out weights the increase in thelow temperature region.

Using standard cell instances with different transistor thresholdvoltages is one example of making netlist changes without significantlyaltering the placement solution. Other examples include transform asubset of netlist into a functionally equivalent netlist that hasdifferent tradeoff in leakage power, timing, area usages and others.

FIG. 8 illustrates a further synthesis transformation to optimize acircuit design according to one embodiment of the present invention. Inone embodiment of the present invention, a chip has a number of blocks.Thus, the cells in one block have the same transistor threshold voltage.For example, in block 721, cells 713, 715, etc., have the same (oralmost the same) threshold (V_(T1)). The cells in block 723 can have adifferent threshold (V_(T2)).

If the threshold is high, the block is a high threshold block, in whichthe cells have low leakage power and low timing performance (highdelay). If the threshold is low, the block is a low threshold block, inwhich the cells have high leakage power and high timing performance (lowdelay). For example, on one implementation of an FPGA chip, the blocksof cells can be specified as a high threshold block, a low thresholdblock, a medium threshold block, etc. The threshold may be hardwired oradjustable during the computation.

In one embodiment of the present invention, a block of cells can beswitched from one threshold value to another through a designtransformation to reduce the power usage. High threshold blocks are usedto reduce the power usage and low threshold blocks are used to improvethe timing performance. In one embodiment of the present invention, adistribution of various threshold blocks is determined throughoptimizing the power usage while satisfying the timing constraint.

For example, after the timing critical paths and the instances on thetiming critical paths are identified, the timing non-critical instancesin a low threshold block can be moved into a nearby high thresholdblock; and timing critical instances can be moved from a high thresholdblock to a nearby low threshold block.

When a block mostly contains timing non-critical instances, the blockcan be switched from a low threshold to a high threshold. For example,in FIG. 7, the transformation 701 converts the threshold for the block721 from V_(T1) in the design 703 to V_(T2) to generated design 705.

Switch a block from a low threshold to a high threshold may cause someof the paths in or passing the block to become critical (or to violatetiming constraint). Thus, timing critical instances can be identifiedand moved to a nearby block (e.g., block 725) to satisfy the timingconstraint.

In one embodiment of the present invention, the transformation of ablock from one threshold to another is at least partially based on theaverage temperature of the block. For example, a block with a hightemperature is a candidate for transforming into a high threshold blockto reduce the power usage, especially the leakage power. A nearby lowtemperature block may be transformed into a low threshold block, if thenearby low temperature is on a number of the timing critical path passthese two blocks. Such a trade-off transformation on a block basis issimilar to the trade-off transformation on an instance basis illustratedin FIG. 6.

In one embodiment of the present invention, different designtransformations are combined in the optimizing power usage using thetemperature solution.

FIG. 7 illustrates a detailed flow of synthesis transformations tooptimize a circuit design according to one embodiment of the presentinvention. Operation 601 sorts blocks according to the leakage power.Operation 603 processes the block with the highest leakage power.

If operation 605 determines this block can be moved to a location thathas a temperature lower than the current location, operation 607 movesthe block to the location that has a temperature lower than the currentlocation.

If operation 609 determines there is an implementation that has a lowerleakage power than the current implementation, operation 611 determineswhether a timing constraint is violated if the low leakage powerimplementation is used. If the use of the low leakage powerimplementation will not cause timing violation, operation 613 performs atransformation to replace the current implementation with the lowleakage power implementation; otherwise, operation 615 determineswhether there is a different block that can be transformed to meet thetiming constraint if the low leakage power implementation is used.

If a different block can be transformed to meet the timing constraintwhen the low power implementation is used, operation 617 performstransformations for both blocks to reduce the leakage power withoutviolating the timing requirement.

If the block cannot be transformed without violating a designconstraint, operation 619 determines whether to process the next highestleakage power block.

Since the power dissipation estimation is based on the temperaturedistribution, a small perturbation of the temperature distribution doesnot affect the correct selection for elements for transformation. Thus,operations 605-609 can be repeated until the temperature distributionneeds to be updated. The temperature distribution can be re-evaluatedusing the technique described above after a sufficient number of changeshave been made (or if the power dissipation based on the currenttemperature distribution is changed significantly).

One embodiment of the present invention includes timing optimizationusing the temperature distribution of the design.

In one embodiment of the present invention, a leakage sensitivity map isconstructed based on the temperature distribution. The leakagesensitivity map shows the sensitivity of leakage power to thetemperature change (e.g., d P_(leakage)/d T) for various locations onthe chip. The leakage sensitivity map can then be used to identify theinstances or blocks for transformations to reduce leakage power. Forexample, the instances with large sensitivity of leakage power to thetemperature change can be selected for transformation (e.g., placementchanges, implementation changes, threshold changes, etc.) before theinstances with small sensitivity of leakage power to the temperaturechange are selected for transformation. Thus, optimizations can beperformed for the cells or blocks with the greatest sensitivity.Instances with low temperature sensitivity could then be used topreferentially improve timing. In one embodiment of the presentinvention, the sensitivity of leakage power to the temperature change isalso included in the objective function (e.g., during placement orsynthesis transformation) for the optimization of a design goal.Further, the sensitivity of leakage power to allowable transformationcan be computed to select the cells (and blocks) and transformations.For example, to select a transformation, the changes in leakage powerfor a number of candidate transformations on a number of candidate cellscan be evaluated. The candidate transformation on the candidate cellwith most reduction in leakage power can be selected for thetransformation. Such a selection process can be repeated to performtransformations that are most effective in the reduction of leakagepower.

For a type of cells the delay from an input to an output and thetransition time at the output of the cell type can be characterized as afunction of the temperature. Such a function of the temperature can beobtained for the whole library of standard cells and the custom blocksor macros (e.g., through curve fitting measurements for differentstandard cells). One embodiment of the present invention performs timingoptimization using the temperature distribution and the information onthe temperature dependent portion of the delay.

In one embodiment of the present invention, after obtaining theself-consistent temperature distribution and power dissipation, a statictiming analysis (STA) is performed to identify the critical paths in thedesign.

Using the results of STA, the top N critical paths and the cellinstances that the critical paths traverses can be identified. Becausethe cell delay and transition times increase with temperature, theworking temperature of the instances can be reduced by making netlistand/or placement changes (e.g., moving high delay elements from hightemperature regions to low temperature regions) in a way similar to thepower minimization. Further, low delay implementations, which may occupya larger area, can also be used. For example, low transistor thresholdcells can be used in low temperature regions to improve timing withleast increase in power usage.

According to one embodiment of the present invention, one technique toimprove the timing involves placement changes. For example, cells withexcessive cell delay and transition times due to elevated temperaturescan be moved from high temperature regions to low temperature regions.

Placement changes over a large distance may involve significant changeto the circuit design, which may not be desirable at a late stage ofdesign (e.g., after the placement and routine solution is obtained). Inone embodiment of the present invention, techniques to improve thetiming involve netlist changes.

For example, a cell at a high temperature region may be on a number ofpaths, among which one path (or a portion of the paths) is timingcritical. The cell can be replicated so that the replicated cell can beplaced in a low temperature region for the critical path. Such atechnique can be used to reduce the impact of placing a cell into a lowtemperature region.

Further, for example, the temperature distribution can be used to selectcells on the low temperature regions of the critical paths so that theselected cells can be replaced with standard cells or custom cells thathave low delays (e.g., low transistor threshold voltage cells).

In one embodiment of the present invention, a delay sensitivity map isconstructed based on the temperature distribution. The delay sensitivitymap shows the sensitivity of delay to the temperature change for variouslocations on the chip. The delay sensitivity map can then be to identifythe instances or blocks for transformations to reduce delay. Forexample, the instances with large sensitivity of delay to thetemperature change can be selected for transformation (e.g., placementchanges, implementation changes, threshold changes, etc.) before theinstances with small sensitivity of delay to the temperature change areselected for transformation. Thus, optimizations can be performed forthe cells or blocks with the greatest sensitivity. In one embodiment ofthe present invention, the sensitivity of delay to the temperaturechange is also included in the objective function (e.g., duringplacement or synthesis transformation) for the optimization of a designgoal. Further, the sensitivity of delay to allowable transformation canbe computed to select the cells (and blocks) and transformations. Forexample, to select a transformation, the changes in delay for a numberof candidate transformations on a number of candidate cells can beevaluated. The candidate transformation on the candidate cell with mostreduction in delay can be selected for the transformation. Such aselection process can be repeated to perform transformations that aremost effective in the reduction of delay.

Further, in one embodiment, both the leakage sensitivity and delaysensitivity are used for the optimization of combined goal of reducingleakage and delay. For example, sensitivities of leakage power and delayto temperature can be combined to generate a sensitivity indicator,which can be sorted for cells (and blocks) to select the a cell (orblock) for transformation. Further, to select a transformation, thechanges in delay and leakage power for a number of candidatetransformations on a number of candidate cells can be evaluated. A goalindicator which is a function of the changes in delay and leakage powercan be evaluated. The candidate transformation on the candidate cellwith best goal indicator can be selected for the transformation.

In general, the self consistent temperature solution and the powerdissipation solution can be used to optimize a design goal involving thepower dissipation, the timing constraints and other design parameters(e.g., area usage). For example, the design goal can be the minimizationof total power usage, or total leakage power, under one or more designconstraints, such as timing constraints (e.g., minimum clock period),area usage, routing resource, etc. Further, a design goal can bedesigned to balance the conflicting resource requirements, such as powerusage (or leakage power) and timing performance. For example, the designgoal can be the weighted sum of the power usage (or leakage power) andthe minimum clock period. The design goal can be optimized for thepossible changes of different implementations of cells (e.g., standardcells of different transistor threshold voltages) and differentimplementation of logic functions. Placement variations can also be usedas a variable for the optimization process.

In one embodiment of the present invention, if most of the criticalpaths are in one logical module, the logic module is constrained to bein a region of lower temperature.

After a sufficient number of changes are made to the design of thecircuit, the thermal analysis can be performed again to assure that thethermal map is consistent with the design and to evaluate theimprovement in the timing of the design after the optimization.

Many of the methods of the present invention may be performed with adigital processing system, such as a conventional, general-purposecomputer system. Special purpose computers, which are designed orprogrammed to perform only one function, may also be used.

FIG. 9 shows one example of a typical computer system which may be usedwith the present invention. Note that while FIG. 9 illustrates variouscomponents of a computer system, it is not intended to represent anyparticular architecture or manner of interconnecting the components assuch details are not germane to the present invention. It will also beappreciated that network computers and other data processing systemswhich have fewer components or perhaps more components may also be usedwith the present invention. The computer system of FIG. 9 may, forexample, be a Sun workstation, or a personal computer (PC) running aWindows operating system, or an Apple Macintosh computer.

As shown in FIG. 9, the computer system 101, which is a form of a dataprocessing system, includes a bus 102 which is coupled to amicroprocessor 103 and a ROM 107 and volatile RAM 105 and a non-volatilememory 106. The microprocessor 103 is coupled to cache memory 104 asshown in the example of FIG. 9. The bus 102 interconnects these variouscomponents together and also interconnects these components 103, 107,105, and 106 to a display controller and display device 108 and toperipheral devices such as input/output (I/O) devices which may be mice,keyboards, modems, network interfaces, printers, scanners, video camerasand other devices which are well known in the art. Typically, theinput/output devices 110 are coupled to the system through input/outputcontrollers 109. The volatile RAM 105 is typically implemented asdynamic RAM (DRAM) which requires power continually in order to refreshor maintain the data in the memory. The non-volatile memory 106 istypically a magnetic hard drive or a magnetic optical drive or anoptical drive or a DVD RAM or other type of memory systems whichmaintain data even after power is removed from the system. Typically,the non-volatile memory will also be a random access memory althoughthis is not required. While FIG. 9 shows that the non-volatile memory isa local device coupled directly to the rest of the components in thedata processing system, it will be appreciated that the presentinvention may utilize a non-volatile memory which is remote from thesystem, such as a network storage device which is coupled to the dataprocessing system through a network interface such as a modem orEthernet interface. The bus 102 may include one or more buses connectedto each other through various bridges, controllers and/or adapters as iswell known in the art. In one embodiment the I/O controller 109 includesa USB (Universal Serial Bus) adapter for controlling USB peripherals,and/or an IEEE-1394 bus adapter for controlling IEEE-1394 peripherals.

It will be apparent from this description that aspects of the presentinvention may be embodied, at least in part, in software. That is, thetechniques may be carried out in a computer system or other dataprocessing system in response to its processor, such as amicroprocessor, executing sequences of instructions contained in amemory, such as ROM 107, volatile RAM 105, non-volatile memory 106,cache 104 or a remote storage device. In various embodiments, hardwiredcircuitry may be used in combination with software instructions toimplement the present invention. Thus, the techniques are not limited toany specific combination of hardware circuitry and software nor to anyparticular source for the instructions executed by the data processingsystem. In addition, throughout this description, various functions andoperations are described as being performed by or caused by softwarecode to simplify description. However, those skilled in the art willrecognize what is meant by such expressions is that the functions resultfrom execution of the code by a processor, such as the microprocessor103.

A machine readable medium can be used to store software and data whichwhen executed by a data processing system causes the system to performvarious methods of the present invention. This executable software anddata may be stored in various places including for example ROM 107,volatile RAM 105, non-volatile memory 106 and/or cache 104 as shown inFIG. 9. Portions of this software and/or data may be stored in any oneof these storage devices.

Thus, a machine readable medium includes any mechanism that provides(i.e., stores and/or transmits) information in a form accessible by amachine (e.g., a computer, network device, personal digital assistant,manufacturing tool, any device with a set of one or more processors,etc.). For example, a machine readable medium includesrecordable/non-recordable media (e.g., read only memory (ROM); randomaccess memory (RAM); magnetic disk storage media; optical storage media;flash memory devices; etc.), as well as electrical, optical, acousticalor other forms of propagated signals (e.g., carrier waves, infraredsignals, digital signals, etc.); etc.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

What is claimed is:
 1. A non-transitory machine readable mediumcontaining executable computer program instructions which when executedby a digital processing system cause said system to perform a method forcircuit design, the method comprising: selecting a portion of the designbased on a circuit analysis, the portion of the design having a cellinstance; and changing a transistor threshold value for transformationof the portion of the design to optimize power usage in the portion ofthe design while satisfying a timing constraint for the portion of thedesign, wherein the portion of the design of the circuit is selectedbased on sensitivities of leakage to temperature change.
 2. Thenon-transitory machine readable medium of claim 1, wherein the portionof the design is one of: a path of the circuit; and a logic block of achip that includes the circuit.
 3. The non-transitory machine readablemedium of claim 1, wherein the selecting of the portion of the design isat least partially dependent on a temperature solution for the design ofthe circuit.
 4. The non-transitory machine readable medium of claim 3,wherein the temperature solution is based on heat transfer according toa temperature dependent power dissipation solution for the design of thecircuit.
 5. The non-transitory machine readable medium of claim 4,wherein the method further comprises: determining the temperaturesolution and the temperature dependent power dissipation solution, whichare interdependent and consistent with each other.
 6. The non-transitorymachine readable medium of claim 1, wherein the transistor thresholdvalue is selectively changed for a first portion of a timing path of thecircuit to reduce leakage power, and another transistor threshold valueis selectively changed for a second portion of the timing path tocompensate for at least a part of a timing degradation caused by thetransistor threshold value being changed for the first portion of thetiming path.
 7. The non-transitory machine readable medium of claim 1,wherein the circuit analysis comprises a thermal analysis to determinethe cell instance that is most sensitive to temperature.
 8. A methodimplemented on a data processing system for circuit design, the methodcomprising: selecting a portion of the design based on a circuitanalysis, the portion of the design having a cell instance; andchanging, by the data processing system, a transistor threshold valuefor transformation of the portion of the design to optimize power usagein the portion of the design while satisfying a timing constraint forthe portion of the design, wherein the portion of the design of thecircuit is selected based on sensitivities of leakage to temperature. 9.The method of claim 8, wherein the portion of the design is one of: apath of the circuit; and a logic block of a chip that includes thecircuit.
 10. The method of claim 8, wherein the selecting the portion ofthe design is at least partially dependent on a temperature solution forthe design of the circuit; the temperature solution is based on heattransfer according to a temperature dependent power dissipation solutionfor the design of the circuit.
 11. The method of claim 10, wherein themethod further comprises: determining the temperature solution and thetemperature dependent power dissipation solution, which areinterdependent and consistent with each other.
 12. The method of claim8, wherein the transistor threshold value is selectively changed for afirst portion of a timing path of the circuit to reduce leakage power,and another transistor threshold value is selectively changed for asecond portion of the timing path to compensate for at least a part of atiming degradation caused by the transistor threshold value beingchanged for the first portion of the timing path.
 13. The method ofclaim 8, wherein the circuit analysis comprises a thermal analysis todetermine the cell instance that is most sensitive to temperature.
 14. Adata processing system for circuit design, the data processing systemcomprising: means for selecting a portion of the design based on acircuit analysis, the portion of the design having a cell instance; andmeans for changing a transistor threshold value for transformation ofthe portion of the design to optimize power usage in the portion of thedesign while satisfying a timing constraint for the portion of thedesign, wherein the portion of the design of the circuit is selectedbased on sensitivities of leakage to temperature change.
 15. The dataprocessing system of claim 14, wherein the portion of the design is oneof: a path the circuit; and a logic block of a chip that includes thecircuit.
 16. The data processing system of claim 14, wherein theselecting the portion of the design is at least partially dependent on atemperature solution for the design of the circuit; and wherein thetemperature solution is based on heat transfer according to atemperature dependent power dissipation solution for the design of thecircuit.
 17. The data processing system of claim 16, further comprising:means for determining the temperature solution and the temperaturedependent power dissipation solution, which are interdependent andconsistent with each other.
 18. The data processing system of claim 14,wherein the transistor threshold value is selectively changed for afirst portion of a timing path of the circuit to reduce leakage power,and another transistor threshold value is selectively changed for asecond portion of the timing path to compensate for at least a part of atiming degradation caused by the transistor threshold value beingchanged for the first portion of the timing path.
 19. The dataprocessing system of claim 14, wherein the circuit analysis comprises athermal analysis to determine the cell instance that is most sensitiveto temperature.
 20. A non-transitory machine readable medium containingexecutable computer program instructions which when executed by adigital processing system cause said system to perform a method forcircuit design, the method comprising: selecting a portion of the designbased on a thermal analysis, the portion of the design having a cellinstance; and transforming a transistor threshold value for the cellinstance to optimize power usage in the portion of the design whilesatisfying a timing constraint for the portion of the design, whereinthe portion of the design of the circuit is selected based onsensitivities of leakage to temperature change.
 21. The non-transitorymachine readable medium of claim 20, wherein the transistor thresholdvalue is selectively changed to reduce leakage power, and anothertransistor threshold value is selectively changed to compensate for atleast a part of a timing degradation.